The present invention relates to a ROM (Read Only Memory) table lookup digital filter circuit which uses a ROM table for a multilevel input signal.
Along with the recent proliferation of CDMA (Code Division Multiple Access), it is becoming impossible to use, as transmission digital filters, ROM lookup filters that are used in radio devices for mainstream SCPC (Single Channel Per Carrier system) or TDMA (Time Division Multiple Access). For this reason, a demand has arisen for implementation of an alternative digital filtering method with a small circuit scale.
FIG. 2 shows a digital filter circuit of the first prior art. Referring to FIG. 2, a shift register 21 temporarily stores input 1-bit transmission data and outputs it to a ROM table 23. A counter 22 counts input sample clocks and outputs the obtained count value to the ROM table 23. The ROM table 23 outputs a filter output waveform, which is stored in advance and corresponds to the input data sequence, on the basis of the outputs from the shift register 21 and counter 22. This prior-art digital filter circuit is disclosed in Japanese Patent Laid-Open No. 63-69310.
FIG. 3 shows a digital filter circuit of the second prior art. Referring to FIG. 3, a shift register 31 serially receives transmission data of a plurality of bits and parallelly outputs the bit signals to multipliers 33-1 to 33-n. A counter 32 counts input sample clocks and outputs TAP coefficients corresponding to the count value to the multipliers 33-1 to 33-n. The multipliers 33-1 to 33-n weight the respective bit signals of the parallel outputs from the shift register 31 using the TAP coefficients and then outputs the bit signals to an adder 34.
In a conventional digital filter circuit, when the ROM table lookup filter shown in FIG. 2 is used, a digital filter circuit can easily be constituted. Generally, however, no ROM table lookup filter can be used for a multilevel signal containing a plurality of input bits. Hence, the normal FIR (Finite Impulse Response) filter is normally used. However, since this circuit requires a number of multipliers, the circuit scale becomes large.